How to define a new command
\tkzDefRelativePoint[#1,#2,#3](20:2){C}
This command defines a point from #1 and takes the slope of the line #2 , #3 as x axis.
\tkzDefRelativePoint[A,A,B](20:2){C}
This command defines a point from A and takes the slope of the line A , B as x axis.
This is done here using
\tkzFindSlopeAngle(A,B)
\tkzGetAngle{tkzangle}
\begin{scope}[rotate=\tkzangle]
\tkzDefShiftPoint[A](20:2){C}
\end{scope}
MWE
\documentclass[border=2pt]{standalone}
\usepackage[usenames,dvipsnames,svgnames]{xcolor}
\usepackage{tkz-euclide}
\usetkzobj{all}
\definecolor{fondpaille}{cmyk}{0,0,0.1,0}
\pagecolor{fondpaille}
\color{Maroon}
\begin{document}
\begin{tikzpicture}
\tkzInit[xmax=4, ymax=3]
\tkzAxeXY
\tkzGrid[color=orange]
\tkzDefPoint(1,0){A}
\tkzDefPoint(3,2){B}
\tkzDefShiftPoint[A](20:2){C'}
%
\tkzFindSlopeAngle(A,B)
\tkzGetAngle{tkzangle}
\begin{scope}[rotate=\tkzangle]
\tkzDefShiftPoint[A](20:2){C}
\end{scope}
%
%\tkzDefRelativePoint[A,A,B](20:2){C}
%
\tkzDrawSegment(A,B)
\tkzDrawSegment[color=red](A,C)
\tkzDrawPoints(A,B,C,C')
\tkzLabelPoints(A,B,C,C')
\end{tikzpicture}
\end{document}